Spondylolisthesis is a condition defined by the anterior displacement of a spinal vertebra or the vertebral column in relation to the inferior vertebrae. It can be caused by isthmic or degenerative conditions. For example, spondylolisthesis can be the result of a congenital abnormality, joint damage from a trauma, a vertebral stress fracture caused by overuse of the joint, and/or joint damage caused by an infection or arthritis. Thus, the condition can affect children, young adults, and older adults alike. The condition, which is often located in the lumbar region of the spine, is separated into five grades depending on the degree of displacement, with Grade 1 referring to the lowest degree of slippage (0-25%) and Grade 5 referring to the highest degree of slippage (over 100%). Symptoms can include back pain, buttock pain, pain that runs from the lower back down one or both legs, difficulty walking, and potentially the loss of bladder or bowel control.
Surgical intervention is used when the pain becomes extreme or there is damage to the nerve root or vertebral column. For example, decompression procedures can remove bone or other tissue to reduce pressure from the vertebral column and/or nerves, and spinal fusion procedures can fuse the vertebrae together to stabilize the spine. Posterior approaches (i.e., accessing from the patient's back) that reduce malalignment and stabilize the spine are typically used to correct high-grade spondylolisthesis. However, the posterior approach makes it difficult to access the pedicles of the vertebrae, in which alignment screws are placed to bring the misaligned vertebrae back into proper alignment. In addition, posterior approaches carry a risk of neurologic injury. For example, studies have shown that 35-40% of posterior surgical reduction procedures resulted in neurologic deficits and complications. Furthermore, posterior approaches require significant disruption of the back muscles and do not allow for the release of the deforming forces from anterior ligaments because they cannot be appropriately accessed from the posterior position. Accordingly, further studies have shown that more than half of patients who underwent surgical posterior fusion treatments for spondylolisthesis had fair or poor results.
Spinal arthrodesis (i.e., spinal fusion) can also be provided from an anterior approach. However, current anterior reduction surgery with interbody fusion has a relatively high chance of failure from loss of reduction (i.e., further slippage) if not combined with a second stage posterior surgery. To address this shortcoming, anterior fusion devices generally require supplemental fixation from the posterior side of the vertebrae. However, supplemental posterior stabilization is susceptible to the disruption of back muscles, neural injury, and difficulty accessing anchor points on the vertebra associated with exclusively posterior approaches.